A full-blown trade war between the U.S. and Canada could lead to a recession in both countries, economist says.

Trump said in a post to Truth Social on Monday that one of his first executive orders would be to hit Mexico and Canada with a blanket 25% tariff. Mexico and Canada are the second and third top suppliers of goods to the U.S., according to the Office of the U.S. Trade Representative. The U.S. imports the most goods from China.

A government official says Canada is already examining possible retaliatory tariffs on certain items from the United States should President-elect Donald Trump follow through on his threat to impose sweeping tariffs on Canadian products.